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A STUDY ON THE INTERACTIONS BETWEEN COENZYME Q0 AND SUPEROXIDE ANION. COULD UBIQUINONES MIMIC SUPEROXIDE DISMUTASE (SOD)? RITA PETRUCCI,a ELISABETTA GIORGINI,b ELISABETTA DAMIANI,b PATRICIA CARLONI,b GIANCARLO MARROSU,a ANTONIO TRAZZA,a GIAN PAOLO LITTARRUc and LUCEDIO GRECI*,b aDipartimento di Ingegneria Chimica, dei Materiali, delle Materie Prime e Metallurgia, Università "La Sapienza", via del Castro Laurenziano 7, 1-00161 Roma, ITALY; bDipartimento di Scienze dei Materiali e della Terra, Università, via Brecce Bianche, I-60131 Ancona, ITALY; cIstituto di Biochimica, Università, via Brecce Bianche, 1-60131 Ancona, ITALY. Received 13 July 1999; accepted 12 August 1999 Abstract--An electrochemical study was carried out on 1,4-benzoquinone, duroquinone, coenzymes Q0 and Q10 in the absence and in the presence of molecular oxygen in aprotic (DMF) and protic (DMF/H2O 95:5 (v/v)) media. Water was added because the investigated reactions are deeply influenced by the presence of protons. Q0 and Q10 exhibited a similar electrochemical behaviour. Since Q0 is more soluble in protic medium than the biologically more important analogue Q10, it was chosen as a model for a more detailed investigation. Voltammetric studies of Q0 carried out in aprotic and protic media in the presence of oxygen showed that, besides simple O2- dismutation, theQ0 promoted dismutation of O2- should also be considered. Spectroelectrochemical experiments with the same experimental conditions support the electrochemical results, showing that in the presence of superoxide and in aprotic medium semiquinone Q0- gives rise to a disproportionation equilibrium, while in the presence of water it tends to be reoxidized to the starting Q0 by OOH. EPR measurements are also in agreement with these results. INTRODUCTION In the last decades the inhibition of radical chain reactions involved in the oxidation of numerous substrates has become of prime importance not only in the polymer and food industries, but also in the field of biology and medicine [1]. In this context, ubiquinones, and especially their reduced forms, are increasingly being used as antioxidants for the treatment of a variety of diseases and the modulation of ageing [2]. However, it should be stressed that the principal biological function of ubiquinones remains their role as electron carriers in the mitochondrial respiratory
270 chain [3]. Apart from these classical functions, the intervention of ubiquinones has been invoked to account for superoxide radical formation in normal cell respiration [4], while their reaction with superoxide itself yields the corresponding radical anions and oxygen [5-7]. Starting from a rejuvenated interest in ubiquinones, and in the wake of a previous study in which we examined the mechanism of the dismutation of superoxide radical in the presence of a series of aminoxyls [8], we sought to reinvestigate, under the same experimental conditions, in aprotic and protic media the electrochemical behaviour of 1,4-benzoquinone (1), duroquinone (2) and ubiquinones Qo (3) and Qlo (4) toward superoxide radical. In fact, although the electrochemical behaviour of quinones and ubiquinones has been previously and thoroughly investigated [5, 9-10] the study of their interactions with superoxide [11] is still far from being exhausted, especially because of the lack of comparable data. RESULTS AND DISCUSSION V oltammetric Results Compounds 1-4 all exhibit, in anhydrous deoxygenated DMF, two well defined monoelectronic reduction steps, a first one reversible and ranging between -0.53 V and -0.86 V, and a quasi-reversible second one ranging between -1.31 V and -1.51 1 V. In protic medium [(DMF/H20 95:5 (v/v): the percentage of water was chosen in order to obtain the same experimental conditions for (Qlo), the least soluble compound] all compounds exhibit a positive shift of both the reduction steps, with a much larger shift for the second than for the first (see Table 1).
271
272 The voltammetric study carried out with different concentrations of 1-4 showed that in all cases both the first and the second reduction potentials shift towards more negative values on increasing the concentration. Representative voltammograms for compound 3 are shown in Figures 1 and 2 (solid line). Figure 1 Cyclic voltammograms of Qo in anhydrous DMF 0.1 mol rl TEAP at a GC electrode, scan rate 0.200 V s-1, in the absence (solid line) and in the presence (dotted line) of oxygen. It is worth noting that in anhydrous DMF, oxygen exhibits two monoelectronic reduction steps (see Table): the first one (formation of 020- is a reversible process, while the irreversibility of the second, i.e. the formation of O2== may reflect the fast protonation of the dianion to give H02-, even in dry DMF (promoted by traces of water in the oxygen flux) [8]. Furthermore, the second reduction step generates a broadened anodic peak at a very positive potential ( Ep,, = +1 V), previously attributed to the oxidation of H02 [8]. In protic medium (DMF/HZO 95:5 (v/v)), both cathodic peaks shift toward more positive potentials, this effect being much more pronounced for the second one; in addition a loss of reversibility is observed even in the first step. Compounds 1-4 all were also studied in the presence of oxygen in both aprotic and protic media. The cyclic voltammograms of quinones 1-2 were difficult to interpret because of the ovelap of their reduction peak with that of oxygen: in particular, the first reduction peak of oxygen partly overlaps with the second reduction
273 peak of 1 and completely with the first reduction peak of 2. This overlap was not a problem in the case of Qo (see Fig. 1 and 2) and QlO which showed a similar electrochemical behaviour (see Epc values in Table). Qo was chosen as the substrate for a more careful and thorough voltammetric study being more soluble in both media (even at concentrations greater than 5 x 10-3 mol 1-'), with respect to its biologically more important analogue Qlo. Therefore, results and discussion will be referred to Qo only. Figure 2. Cyclic voltammograms of Qo in DMF/H20 95:5 (v/v) 0.1 mol 1-' TEAP at a GC electrode, scan rate 0.200 V s-', in the absence (solid line) and in the presence (dotted line) of oxygen. In the literature it is often reported that quinone radical anions can be oxidized in the presence of oxygen, with formation of superoxide anion and the corresponding quinones: QO*- + Qo + 02'- this reaction is commonly considered as an equilibrium, and some authors claim that this is thermodynamically shifted to the left [6, 12]. The equilibrium seems to be shifted to the right only in the presence of species that can promote superoxide dismutation [6,11,13,14] or of protons. Although 02'- is a very weak base (pK, of the acid HOO' is 4.69) [15] reaction (4) is very fast [16,17] and shifts the equilibrium of eqn. (2) to the right, increasing the basicity of O2.- to an extent equivalent to that of a very strong base [17]. The high rate constant of reaction (4) is justified by the high oxidising power of HOO* [8,18] but the data reported in the literature cannot be used to evaluate the

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